I'm using workflow look up for user and return data az email address but workflow doesn't send email and give man an error that cant sent email verify the email address!! is there anyone has any idea
I Guess your SMTP server is not running. As @AnandaKrishnan mentioned it could also be a problem relating to the User profile synchronization issue.
If its something relating to your SMTP server, please follow the link, this will give some idea about how to trouble shoot